The systematic water treatment was started in the late 1800s and early 1900s (Tchobanoglous and Burton, 1991). For the last two centuries, water treatment has continually been developed to meet strict disposal standards. Water treatment processes consist of physical, chemical, and biological means. Chemical treatments indicate that the removal or conversion of contaminants is brought about by the addition of chemicals or by other chemical reactions. Flocculation, precipitation, ion exchange, adsorption, and disinfection are the most common chemical treatment methods.
Flocculation consists of four distinct mechanisms: i) compression of the diffuse layer (van der Waals interaction), ii) adsorption to produce charge neutralization (destabilization), iii) enmeshment in a precipitate (sweep coagulation) and iv) adsorption to permit interparticle bridging (complex between particle and polymer with synthetic organic coagulant). Rapid mixing leads to the charge neutralization of colloids/particles through uniform and immediate disposal of chemicals with water. Flocculation which follows the rapid mixing results in the aggregation of particles. Flocculation can occur through three major mechanisms: i) Brownian movement of fluid molecules (perikinetic flocculation), ii) velocity gradient in the fluid (orthokinetic flocculation) and iii) differential settling of different sizes of particles in the water (Vigneswaran and Visvanathan, 1995). Coagulants are classified into three groups mainly used in the real application: i) aluminium sulfate (72%), ii) iron salts (23%), and iii) polyaluminum chlorides (5%) (DeWolfe et al., 2003). Alum and ferric chloride are the most common coagulants. The use of ferric chloride and polyaluminum chloride for water treatment has been increasing over the last few decades.
Metal oxides are often used in advanced oxidation processes which are defined as production of hydroxyl radicals in sufficient quantities to oxidize majority of the complex chemicals present (Gogate and Pandit, 2003). Hydroxyl radicals have an oxidation potential of 2.8 V and exhibits faster rates of oxidation reactions as compared to that using conventional oxidants like hydrogen peroxide or KMnO4. Hydroxyl radicals react with most organic and many inorganic solutes (Hoigne, 1997). Titania (TiO2) is the most widely used metal oxide. Degradation of waste compound proceeds via oxidative (electrophilic) attack of HO. and leads to complete mineralization to yield innocuous CO2 and mineral acids. This process is based on the electronic excitation of a molecule or solid caused by light absorption e.g. UV light that drastically alters its ability to lose or gain electrons and promote decomposition of pollutants to harmless by-products (Molinari et al., 2002). Photoinduced electrons (e−) and positive holes (h+) are produced from TiO2 with UV light. These charged species can further generate free radicals. The highly oxidizing positive hole (h+) is considered to be the dominant oxidizing species contributing to the mineralization process resulting from the TiO2 photocatalysis (Chu and Wong, 2004). The principal advantages of the TiO2/UV process are suitable in water treatment without the addition of large amounts of chemicals, no follow-up treatments (filtration, etc.) are necessary and applicability over a wide range of pH values. Photocatalysis can also be applied in small scale applications such as houses (water sterilizer, air sterilizer), automobiles (frost-preventing glass, anti-germ seat) and sterilizing devices. It can also be used to get rid of germs and malodors from polluted air. The photocatalytic technology can keep air and water clean, using ultraviolet (UV) or sunlight. Therefore, the demand of titania is increasing more and more.
There is a need for a development of a new coagulant to induce metal oxides in water treatment. This will lead to an efficient and economical water treatment. Also, this will meet the demand of metal oxides used in many applications.